k
up hum. I've heard a medium wave transmitter getting picked up if the setup=
happens to resonate at that frequency.
In the BBC training school they retold a story of a radio outside broadcast=
in the country where they clipped a balanced line mic using crocodile clips=
to a barbed wire fen for half a mile and it worked.
Impedance is a tricky one to tackle as the real impedance of a mic or input=
is rarely what it says on the device. What is often quoted should be the
"optimum noise impedance" which gives the best sound match for system noise=
but not necessarily for picked up interference.
On very long cables, you need to match the mic, the cable and the input
impedances for the best performance, but 15 metre cables (theoretically
around 150 Ohms impedance) will probably hold up for most medium impedance=
(10 to 20 KOhms) connections
Another curious noise is cable microphony when tapping a cable can be picke=
d
up. Try it without a mic plugged in.
Sorry I've rambled on without answering your second question. If you have
trouble with 15 metre cables - which I wouldn't expect - you could use a
preamp to get a lower sending impedance, more important than a higher signa=
l
level.
With my Brinibox, using cheap medium impedance mics, I used mic transformer=
s
to get a balanced line feed on long cables as my mixer was already balanced=
line. This got over the problem of my humming house.
David Brinicombe
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